1. Field of the Invention
The present invention relates to a semiconductor device having a capacitor which utilizes an oxide dielectric substance, such as a perovskite dielectric substance.
2. Description of the Related Art
Integrated circuits (ICs), which have been remarkably developed in recent years, include transistors and capacitors integratedly arranged in one semiconductor chip. One of the factors which determine the cost of an IC is the size of semiconductor chips. As the chip size is smaller, the cost of an IC becomes lower. In order to make the chip size smaller, it is important to decrease the area occupied by elemental devices in a semiconductor chip. Among the elemental devices, capacitors have the greatest occupation rate of area relative to the total chip area. Therefore, the chip size can be greatly decreased by making the area of each capacitor smaller.
Generally, an MIM (Metal Insulator Metal) capacitor structure, in which a dielectric thin film is interposed between electrode thin films, is adopted in a capacitor to be used in an IC, for the purpose of miniaturization and so forth. The capacitance C of an MIM capacitor is expressed by C=(.epsilon..sub.r .times..epsilon..sub.0 .times.S)/d, where .epsilon..sub.r is the relative dielectric constant of the dielectric film, .epsilon..sub.0 is the dielectric constant of vacuum, S is the electrode area, and d is the thickness of the dielectric film.
Therefore, it is necessary for decreasing the electrode area of a capacitor to increase the relative dielectric constant of its dielectric film or to decrease the thickness of the film. However, there is a limit in the thickness of the dielectric film, since leakage current becomes greater as the dielectric film is thinner. Furthermore, where the thickness of the dielectric film is small, it is difficult to control the thickness in a manufacturing process, thereby bringing about irregularity in the characteristics of capacitors.
In relation to increase in the relative dielectric constant, it has been proposed to use, as the material of the dielectric thin film, a perovskite dielectric substance, which has a high dielectric constant, in place of SiO.sub.2, SiN, or SiON conventionally used. Strontium titanate (STO) expressed by a chemical formula of SrTiO.sub.3 and barium strontium titanate (BSTO) expressed by a chemical formula of BaSrTiO.sub.3 are representative ones of the perovskite dielectric substances.
A semiconductor memory device, represented by a DRAM, memorizes data by storing electric charge in a number of capacitors. Therefore, decrease in the area of each capacitor greatly facilitates increase in degree of integration level. In other words, it is effective for improving the degree of integration level to increase the dielectric constant of the dielectric film.
In a GaAs monolithic microwave IC (MMIC) which has attracted attention due to application to a potable telephone, a capacitor having a large capacitance is sometimes used for decoupling between a power supply and an impedance matching circuit. In such an IC, the chip size can be decreased by increasing the dielectric constant of a dielectric film and decreasing the electrode area.
FIG. 2 is a cross sectional view showing a conventional capacitor employed in an MMIC. This capacitor is manufactured as follows.
An SiO.sub.2 film 2 is formed on a GaAs substrate 1. A first wiring layer 3 made of Au or the like is formed on the SiO.sub.2 film 2 by means of a lift-off method.
Then, a metal film to be a lower electrode 4 is formed on the first wiring layer 3 by means of a lift-off method. An STO dielectric film 5 is deposited on the lower electrode 4 by means of a reactive sputtering method at about 300.degree. C. An upper electrode 6 is formed on the dielectric film 5 by means of a lift-off method. The STO dielectric film 5 is patterned by means of wet etching, using the upper electrode 6 as a mask.
Then, an SiO.sub.2 film 7 as an inter-level insulator is deposited and a contact hole is formed. A second wiring layer 8 made of Au or the like is then formed.
In such a capacitor, its characteristics are degraded due to oxidation of those surfaces of the lower and upper electrodes 4 and 6, which are in contact with the dielectric film 5. As the material of the electrodes 4 and 6, Ti is used, since it has an excellent adhesion. In this case, the upper surface of the lower electrode 4 is oxidized in the step of depositing the dielectric film 5, which is carried out in an atmosphere containing oxygen. The lower surface of the upper electrode 6 is oxidized when the upper electrode 6 is formed.
If those surfaces of the electrodes 4 and 6, which are in contact with the STO dielectric film 5, are oxidized, a metal oxide produced by this oxidation act as a dielectric substance having a low relative dielectric constant. In this case, a capacitor having a relatively low capacitance due to the metal oxide is connected in serial to a capacitor having a high capacitance due to the STO film, as in an equivalent circuit. As a result, the total capacitance of a capacitor formed between lower and upper electrodes 4 and 6 becomes low.
Generally, the oxide layer formed in the lower part of the upper electrode 6 is thinner than the oxide layer formed in the upper part of the lower electrode 6. As a result, where the upper and lower polarities of applied voltage are reversed, the electric characteristics may differ. In order to decrease this difference, an additional oxide is produced in the upper electrode on purpose so that the oxide layers in the lower and upper electrodes 4 and 6 balance with each other. However, this brings about the decrease in dielectric constant described above.
It is known that the crystallinity of an STO film is improved and its dielectric constant is increased by heat-treating the film at a temperature as high as 600.degree. C. or more. Therefore, when capacitor electrodes are selected, their heat resistance should be taken into account. In Jpn. Pat. Appln. KOKAI Publication No. 59-97818, nitride, silicide, or carbide of a metal, such as Ti, Ta, Nb, Hf, Zr or the like is used, in place of a single metal, such as Ti or the like, as the material of an electrode, in order to improve the heat resistance of the electrode. However, it has been found that where these materials are applied to a capacitor employing an oxide dielectric film, such as an STO film, and having a high capacitance, their oxidation resistance is not enough so that a metal oxide is produced and the capacitor is decreased in its capacitance. Furthermore, these materials have a high electric resistance and do not suit for the electrode material of a capacitor used in an MMIC.
Precious metals, such as gold and platinum, have an excellent oxidation resistance. However, these materials are not appropriate for an IC, since they are expensive, and are hardly patterned by means of a reactive dry etching method. Furthermore, they are thermally unstable, and properties, such as dielectric dispersion, are irregular. FIG. 5 is a graph showing dielectric dispersion relative to bias voltage in a capacitor having an STO dielectric film interposed by Pt electrodes. In this graph, different three marks respectively represent different three measured points in one plane. As shown in FIG. 5, hysteresis occurs relative to the bias voltage at respective measured points. This means that two dielectric dispersion values are produced in relation to one applied voltage, whereby no stable electric properties can be obtained.